The present invention relates to a thermosetting coating composition. More particularly, it pertains to a thermosetting coating composition capable of forming an organic film with excellent anticorrosion properties which contains an inorganic cation-exchanger and which is suitable for use in the production of precoated metals (PCM).
Precoated metals, which are mainly used in the automobile and electrical appliance industries, are required to have various properties. For example, they should have not only good anticorrosive properties and weathering resistance but also good press formability and improved productivity in the preparation and application of coating. Furthermore, it is desirable that precoated metals or the production thereof should not cause environmental (air, water, etc.) pollution.
Many coating compositions which have been used to form an anticorrosive coated film in the production of precoated metals, particularly precoated anticorrosive steel sheets, contain a chromium (VI) compound which forms a corrosion-resistant passive film on the substrate metal. However, such coating compositions have the problem that elution of toxic chromium from the resulting coated film inevitably occurs, thereby causing water pollution. Furthermore, the coated film from which chromium has been eluted has a greatly degraded level of anticorrosion power and therefore is not expected to exhibit long-lasting anticorrosive properties.
In order to avoid water pollution caused by chromium-containing coating compositions, there is a need of developing chromium-free coating compositions. One class of such coating compositions contains silica (SiO.sub.2) and/or other metal oxides in the form of fine or colloidal particles in place of a chromium (VI) compound, as described in Japanese Patent Applications Laid-Open Nos. 62-57470(1987), 63-123472(1988), and 1-167381(1989), and European Patent Application Publication No. 0 307 996.
However, the anticorrosion power of these silica-containing coating compositions is unsatisfactorily lower than that of conventional chromium-containing coating compositions. Therefore, it is still desired to develop a chromium-free coating composition having a satisfactory level of anticorrosive properties which are comparable to those of chromium-containing compositions.
It has been proposed to incorporate a melamine cyanurate compound, which is an adduct of melamine and cyanuric acid, into a coating formed on steel sheets. Japanese Patent Application Laid-Open No. 2-149699(1990) discloses a steel sheet having an electroplated coating of zinc or a zinc alloy which is formed from a zinc or zinc alloy plating bath containing a melamine cyanurate compound dispersed therein.
Japanese Patent Application Laid-Open No. 3-28277(1991) discloses a metallic finish coating in which a melamine cyanurate compound is added to the uppermost clear layer in order to provide the metallic finish coating with an attractive pearlescent appearance similar to that of pearlescent coatings.
However, it has not been proposed to use a melamine cyanurate compound in inorganic particle-containing, organic resin-based coating compositions, particularly for use in the production of precoated metals.